1. Field of the Invention
The present invention relates to a passive optical network (PON) for providing a large amount of data at high speed to subscribers.
2. Description of the Related Art
Currently, data service is mostly provided to internet service subscribers through xDSL (Digital Subscriber Line) (ADSL, VDSL), cable modems, dial-up modems, etc., at speeds of 56 kbps and higher. However, in order to provide various services henceforth—such as large quantities of visual information service, real-time VoD (Video on Demand) service, high quality broadcasting service, and so forth—to subscribers, a data transmission speed of about 100 Mbps is required. Thus, it is impossible to provide the services using existing telephone lines, UTP cables, etc. To address this need, the necessity of construction of optical subscriber network using optical communication is increasing rapidly. A passive optical network (PON) has been suggested and is being developed as a new economical way of forming an optical subscriber network.
The PON comprises at least one OLT (Optical Line Terminal), a plurality of ONUs (Optical Network Units) or ONTs (Optical Network Terminals), and a passive optical coupler. Such PONs have largely been classified into ATM-PONs (Asynchronous Transfer Mode-PONs), Ethernet PONs, and WDM-PONs (Wavelength Division Multiplex-PONs), according to their implementation methods. Further, a new research in the area of CDMA-PON employing CDMA is growing.
FIG. 1a to 1d are schematic views illustrating the general constructions of PON systems according to the prior art.
First, FIG. 1a shows a schematic view of an ATM-PON employing an ATM (Asynchronous Transfer Mode), in which an upstream communication is performed with the transmission of ATM cells at a wavelength of 1310 nm at 155 Mbps and a downstream communication is performed with the transmission of ATM cells at a wavelength of 1550 nm at 155/622 Mbps.
FIG. 1b shows a schematic view of an Ethernet PON employing an Ethernet mode according to the prior art, which has the same upstream and downstream wavelengths as those of the ATN-PON, but uses Gigabit Ethernet signals of 1.25 Gbps for both upstream and downstream signals. Also, the ATM PON uses cells of fixed length, while the Ethernet PON uses Ethernet frames having variable length.
FIG. 1c shows a schematic view of a WDM-PON employing a WDM (Wavelength Division Multiplex) according to the prior art. As shown, the wavelengths used for transmission and receptions are assigned to each ONU individually. Therefore, the WDM-PON uses wavelength multiplexer/demultiplexers not as a passive optical coupler as in the ATN-PON and the Ethernet PON.
Finally, FIG. 1d shows a schematic view of an optical subscriber network employing CDMA (Code Division Multiple Access) technology for both upstream and downstream communications, in which the transmission speed of upstream and downstream data is about 10 Mbps.
The ATM-PON and the Ethernet PON of the prior art use TDM (Time Division Multiplexing) technology for downstream communication and TDMA (Time Division Multiple Access) technology for upstream communication. In the case of the downstream signals, data are transmitted according to a broadcasting method to avoid signal collisions. However, in the case of the upstream signals, the same wavelength is used when two or more ONUs or ONTs transmits their signals to an OLT simultaneously, so that signal collisions may be caused in the passive optical coupler. Therefore, the ATM-PON and the Ethernet PON have to use a very complicated MAC (Media Access Control) protocol in order to address this problem.
In addition, as the distances between the OLT and each ONU or ONT are different from each other, various optical signals of different strengths are inputted into an optical receiver in the OLT. As such, a BMIC (Burst Mode IC) is required to receive the various optical signals in stabilization. Also, an optical transmitter in each ONU or ONT requires a BMIC capable of operating the transmitter only in the case of signal transmission.
As described above, the ability of the ATM PON and the Ethernet PON to receive a guaranteed bandwidth is largely restricted.
Meanwhile, in the case of the WDM-PON, since a MAC is not used, operation of the PON system is simpler, and a broad bandwidth transmission can be guaranteed; however, it is difficult to produce optical transmitter/receiver modules at a low cost. As a result, continuous studies and development have been made to produce economical optical transmitter/receiver modules.
Lastly, in the case of the conventional optical subscriber network employing CDMA, the use of a MAC is not required as CDMA scheme is applied to upstream communication. However, since the CDMA technology is also used for downstream communication in the optical subscriber network, the construction of the ONU or ONT and the OLT is complicated, thereby increasing the manufacturing cost. Further, in the conventional optical network employing CDMA, a switch in the OLT must divide data according to each subscriber before the transmission of the data, thereby complicating the operation of the OLT.